[PATCH] Constrain swap overload for std::optional (LWG 2748)

[Jonathan Wakely]

This implements the resolution to LWG 2748 which was approved the
other day at the WG21 meeting. I think the resolution is wrong,
because as the test shows it means that optional<NonSwappable> is
swappable in some cases. I've raised that with the LWG and will
probably create a new issue for it, but in the meantime this
implements what the spec says.

	* doc/xml/manual/intro.xml: Document LWG 2748 status.
	* include/std/optional (optional<T>::swap): Use is_nothrow_swappable_v
	for exception specification.
	(swap(optional<T>&, optional<T>&)): Disable when T is not swappable.
	* testsuite/20_util/optional/swap/2.cc: New test.

Tested powerpc64le-linux.

This only affects experimental C++17 stuff, so I'm committing it even
though we're in Stage 3.

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